Fief

The Fief is the being or entity that mints coins, according to Saradomin in the previous issues of the God letters[1][2]. He does not give more detail as to who or what a fief is, but judging from the meaning of "fief," it is likely to be one of the human kingdoms (e.g. Misthalin).

The Fief is mentioned in the 14th issue of the God letters:

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Bite9611111: We where hoping that you would make an update that allows crafters to make gold coins from gold bars

Saradomin: Forgery is a serious crime, and only those coins minted by the respective Fief are legal tender. If forgery were not a serious crime here on RuneScape the economy would surely have been destroyed by a certain spell known as high level alchemy by now...

Apparently, different fiefs are responsible for producing their respective kingdom's currency. This may be a joke on Jagex's part, because a fief is a grant of money from a king to a vassal, and it is printed money (not how money is minted from). However, the literal definition of 'fief' is 'a piece of land held under the feudal system'. Therefore, it is likely that when 'fief' is mentioned, it refers to one of the human kingdoms.